Conventional influenza virus vaccines induce antibody responses of limited breadth. Whether mRNA-based influenza virus vaccines can induce a superior germinal center (GC) response in humans remains unclear. Here we assessed B cell responses in an observational study of cohorts of healthy young adults receiving a licensed, split-virion or investigative mRNA-based quadrivalent seasonal influenza virus vaccine over two consecutive seasons. mRNA-based vaccines consistently elicited higher antibody titers and frequencies of memory B cells. In the draining lymph nodes, mRNA vaccination stimulated sustained GC reactions that persisted for at least 26 weeks after vaccination in 5 of 13 participants across the two seasons. Proteomic analysis of serum IgG repertoire showed that mRNA vaccination increased the number of vaccine-elicited serum IgG clonotypes and promoted intraclonal expansion within pre-existing clonotypes. B cell lineage analyses further indicated that expanded serum clonotypes map to GC B cell-associated sub-branches, consistent with ongoing GC-driven evolution underlying intraclonal expansion. This repertoire remodeling was accompanied by increased binding breadth against antigenically divergent influenza viruses. These findings reveal a key role for persistent GC responses in broadening the repertoire of vaccine-induced antibodies.